This invention relates to optical disk storage units, and more particularly, to an improved servo loop for the access and tracking system of such a unit. Optical data storage units which utilize a disk to optically store information have a servo system which controls the positioning of the read/write head to provide direct access to a given track of data. U.S. Application Ser. No. 438,133, filed Nov. 1, 1982, Stahl et al, now abandoned, and corresponding PCT application, International Publication No. W084/01849, published May 10, 1984, show a servo system for an optical disk storage unit.
Two tracking servo systems are employed to enable accurate reading and writing of data from or to data tracks. A coarse servo tracking system radially positions the optical head over a desired data band on the disk. A fine servo tracking system then optically (through the use of tracking mirrors attached to a galvonometer) positions the read/write beams on a desired track(s) within the data band.
Galvonometer controlled tracking mirrors have a limited radial range, hence the necessity for a coarse servo system that: (1) radially positions the optical head over a desired data band where the desired data track is located (coarse seeking), and (2) maintains the head in this desired position (coarse tracking). The coarse tracking servo accurately maintains the head with respect to a precision coarse servo track previously written on the disk, which coarse servo track is adjacent to the data band of concern.
The detection system employed in a drive of this type is a position detection system wherein a position error signal is generated indicating the position of the optical head with respect to a given data band. The position error signal has a sawtooth waveform including ramps as the head crosses each data band and a steep slope as the head crosses data bands.
For a number of reasons, a single velocity signal which represents true velocity over its entire range is not available in such a system. For this reason, it is necessary to derive a plurality of velocity signals, for low, medium and high head velocities.
It is an object of the present invention to generate a true velocity feedback signal from a detected position signal for use in a velocity servo system, which velocity servo system radially moves an optical head from over a source track to over a target track of an optical disk.